Malignant transformation of cells by viruses often is induced by a virus-related aprotein which plays no role in virus reproduction. Sequence analysis of viral genomic regions responsible for transformation has allowed the projection of the amino acid sequences of two such proteins, and antibodies against certain of these sequences were developed. One antibody has allowed the tentative identification of the protein responsible for avian myeloblastosis, and another selects the protein encoded by MC29 virus, and a similar protein in nontransformed cells. The location of these proteins within the cell has been determined, and possible glycosylation and phosphorylation modifications were examined. In a continuation of earlier experiments on Rous sarcoma cells, certain metabolic differences related to transformation have been examined in detail. Although Na-K-ATPase was shown to be coupled to glycolytic ability, no substantial changes in the activity or efficiency of Na-K-ATPase were noted despite the increased glucose consumption and glycolysis of transformed cells. An increased production and utilization of ATP by transformed cells has not yet been explained.